So, if I can hack an F5 for 4K, can I also get the F55’s color gamut?

So, we have seen that it is possible to trick an F5 into thinking its an F55 by altering an all file from the F5 and adding some F55 4K settings. This enables 4K internal recording and 4K output over HDMI on the F5. Internal 4K is one of the key differences between the lower cost F5 and the much more expensive F55. Another major difference is that the F55 has a global shutter so no CMOS image skew or other rolling shutter artefacts and the F55 has a larger colour gamut allowing better colour rendition and capture.

A question that has been asked is: Well if we can get 4K, can we also enable the larger colour gamut? One thing we do know is that the sensor used in the F55 is different to the sensor in the F5 as the sensor is replaced if you upgrade your F5 to an F55.

A cameras gamut is determined more by the sensors colour filters than the recording gamut. The recording gamut is like a bucket, the sensor a scoop. If the scoop isn’t big enough you won’t fill the bucket.

The color filters on the F55 are very different to those in the F5, so the F55 can capture a much greater gamut than the F5.

If you think about it, if you hold a red gel up infront of your eyes you will only see an extremely narrow colour gamut, just a single narrow part of the red spectrum. Imagine if you have a red, green and blue filter, you will now see a bit of red, a bit of green and a bit of blue. But you might only see a very narrow part of the full blue spectrum or a very narrow part of red or of green, you won’t see the full spectrum or a large gamut, just narrow slithers of it. The trick is to make filters that are wide enough and with the righ charcteristics to pass all of the R, G and B spectrum but sharply cut off unwanted colors, infra-red or UV at the exactly the right point. This is very hard to do. So the quality and accuracy of the color filters determines both the gamut and the precision of the colors that the camera can capture.

In practice it can be hard to see this difference as none of the monitors available today can show the full gamut that the F55 can capture so you can’t directly see it. But it does make a difference in post as the F55 is able to separate subtle hues more accurately and capture an extended tonal range, for example very subtle differences in skin tones that may be lost on a camera with poorer filters. This means when grading you are able to draw more tonal information out of the image when you transform the color space in to Rec 709 or DCI-P3 and it results in a more natural looking image.

The F5’s sensor gamut is probably somewhere around the size of DCI P3, maybe a bit bigger, but it’s clearly not as big as the F55’s. In addition the colour precision is not as great so some subtle tones are lost. It’s not a massive difference and the F5 does a great job. It’s not something that can be changed with software, it’s all down to the sensor hardware. The F5 just can’t fill the S-Gamut recording bucket so by using S-Gamut your wasting a lot of data. By using a smaller recording gamut like S-Gamut3.cine you can more effectively fill the bucket and make better use of the data available to you.

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Listen Up, XDCAM USERS!

Hi Guys and Gals.

So, IBC is just a few days away. I’m packing my bags, ready to go and I’m really rather excited. I’m under NDA so can’t reveal anything and maybe even writing this post will get me in trouble.

Unless you have completely had your head in the sand (or have been working hard) you will have probably seen the leaked picture of a new XAVC badged camcorder that has become known as the FS700 MK2 by many. You might also have seen a teaser video from Sony featuring several well known DP’s and bloggers talking about a new camera, if you haven’t click here. It doesn’t tell you much, but talks about what some people would like in a new camera, with the tag line “Be first to discover true freedom of expression”. So I don’t think I’m revealing any secrets by saying that it looks like there will be a major camera launch at IBC and it’s one that’s going to rock the boat a bit (well quite a LOT actually).

In recent months we have seen Sony release a new line of PXW XDCAM cameras that have Sony’s new and really very good XAVC codec. Yes, it’s a little confusing but these are still XDCAM cameras…. XDCAM meaning digital recording on to SxS or Optical Disc. So far we have seen the diminutive PXW-X70, the mid range 1/3″ PXW-X160 and X180 as well as the update to the PMW-300 to add XAVC. So IBC will be a great time to see the full range of XAVC cameras and I’m sure we can expect more news about XDCAM and XAVC in general.

There has also been a lot of commotion around the “hack” that allows owners of the PMW-F5 to get internal 4K recording. So far Sony have only responded to this with a brief “we do not approve and it may invalidate your warranty” type statement. I’m hoping we will get clarification from Sony over what they will do about this in future firmware updates. Will they leave it (doubtful), block it (likely) or just give F5 owners a 4K upgrade path (the BIG unknown) other than getting the entire camera upgraded to an F55. Again I’m sure we will get lots of news on the development of the PMW-F5 and F55 cameras.

I’ll be there at the show helping out on the Sony booth (no, I am NOT a Sony employee. I just get asked to work the booth to share my practical experience with the cameras). So I won’t get a lot of time to reveal all the secrets when they are announced here on the blog, but if you keep an eye on my twitter feed ( @stormguy ) I’ll try to tweet the news as fast as I can. The Sony press event is on Friday morning (12th Sept) so you can expect to see a flood of very exciting news immediately after that.

 

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From Script to Screen. SEVA Fest 2014 film makers festival, South East Virginia.

I’m really please to be involved with the Mid-Atlantic Conference for film professionals which will be held in SouthEast VirginiA. . I’ll be running workshops along side Bruce Logan, the man that shot the blowing up the Death Star in the original Star Wars movie, was a cameraman on Tron, Batman Forever and many other films. Bruce, as well as bing a very talented DP is also a script writer, producer, director and colourist. Actor and Director Michael Copon, known for his roles in Power Ranges and Scorpion King – Rise of a Warrior  will  also being running sessions during the course of the 3 day event.

SEVA  is open to anyone with a desire to network with other film professionals, discover new talent, and to better learn the creative technologies, techniques, and tools of the industry.

The inaugural SEVA Fest will be held at the Chesapeake Conference Center from October 17 through October 19. It will feature workshops and panel discussions hosted by Bruce Logan, Alister Chapman, and Michael Copon with the emphasis of storytelling using today’s modern technology. It will also feature a short film competition, to be held at the Roper Theater in Norfolk.

Workshops will include: Script to Screen with Bruce Logan, Painting and setting up digital cameras, by me. Acting and Producing with Michael Copon. There will be lots to learn, a great chance to meet some amazing people and hopefully a lot of fun.

There’s student and early bird pricing so jump on over to the  SEVA website for full details: http://www.sevafest.com/

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The RIGHT way to get good timecode sync with multiple cameras.

So, you have a multi camera shoot and you want to have the timecode in perfect sync between all of the cameras. For a start lets assume we are talking about pro cameras that actually have timecode in and out, because without a way to connect an external timecode source, getting truly accurate TC sync is all but impossible. The other thing you need if you want REAL timecode sync is genlock (or an Arri Alexa with an Ambient Lockit box).

The most common way used to get timecode sync across multiple cameras is to simply connect a timecode source to the timecode in of the cameras. This can be done with cables or wirelessly. This is a method I’ve used many times and it works….. kind of. Actually this is NOT the best way to get good timecode sync, but it’s probably the most commonly used method. It is especially problematic when you have very long takes, say shooting a rock concert without stopping between songs.

Here’s the problem.

When you hit record on the camera the timecode MUST increment by 1 frame every time you record a new frame, regardless of what the timecode on the TC in is doing. Every frame MUST have a unique TC number. So, if the cameras sync clock is running a tiny bit faster or slower than the TC clock of the external source, the cameras TC will slowly drift out of sync with the external TC until you stop recording at which time the cameras TC will re-sync with the external TC. On long takes this may result in a loss of sync between the external TC and the TC generated by the camera. Often this isn’t more than a few frames, but on a music shoot or where you have sync sound, being a few frames out can be a real pain.

Timecode will not synchronise a camera, it will not pull the cameras frame rate into sync with the external TC clock. Unless it is an Arri Alexa and you are using an Ambient Lockit box as Ambient can pull the Alexa’s clock into sync via a special “Tune” pin. The only thing that will alter the cameras actual frame or sync rate is genlock. So if you want the cameras to truly stay in sync you must genlock them to a common sync source.

Now I know that very often this is not possible, especially with remote or mobile cameras. That’s why companies such as Ambient include a sync output that you can connect to the cameras genlock in on their wireless TC boxes, so you can genlock the camera to bring it in to true sync with the external clock as well as feeding it sync TC.

If your working with a single camera and a sound recordist, rather than having the soundie feed TC to the camera, the more accurate way is to use the camera as the TC source and send the TC to the sound recorder. The sound recorder doesn’t have a frame rate as such so the TC does not need to be in sync with the sound recorder in the same way as it should be in sync with the cameras actual frame rate. A sound recorder can have time periods shorter or longer than a frame, a video camera cannot. By sending the TC from the camera to the sound recorder you will eliminate sync drift between the TC and the actual video frame count during longer takes.

I know this is not how it’s done in practice. In most cases TC is sent to the camera and in most cases genlock isn’t used. Probably because this is the easiest way to do things. Most of the time, if the takes are shorter than 10 minutes or so, you won’t see any issues. But if you really want accurate TC over long takes you need to do it properly and either genlock the cameras or use the camera as the TC source on a single camera shoot.

 

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Hack to get internal 4K on the Sony PMW-F5 CAUTION THERE MAY BE ISSUES!!!!

STOP PRESS: It’s been found that this modification is also changing the exposure and dynamic range! See bottom of post.

Well, Paul Ream over at http://extrashot.co.uk/ has worked out how to make the F5 record 4K internally. It’s actually pretty straight forward. The camera can save a file called an “All File” on an SD card to memorise the way it’s set up. By mdifying this file you can trick the F5 to go into 4K or UHD frame size instead of the usual HD or 2K. First you have to modify line 15o of an existing All File (they are simple text files) where the frame rate and frame size are saved using a text editor, putting in the frame size you want. Then delete line 1 where the files MD5 checksum is stored. Save the file. Next determine the checksum of the file, then add the new checksum to line 1 and finally save the file to the SD card and read it in to the camera. By loading this modified (hacked???) all file in to the camera you can enable 4K and UHD recording to the SxS cards.

It’s certainly created a stir. Many F55 owners think they have been short changed because of this hack and of course most F5 are very happy. It will be interesting to see if any other F55 features can be unlocked on the F5 such as 4K output or 4K playback. Right now the hack is a little limited as you can’t even playback files to see if they are OK. Full details over on Extrashot in the latest podcast…. if you can grind your way through the bit on shoes and iphones without nodding off first.

UPDATE: So, now a few people have tried this, there have been a couple of reports of the dynamic range of image clipping and some other issues. I’ve had a quick look on an F5 (not mine, I’m travelling) and there is most definitely something odd going on as the pictures are darker and clip earlier in 4K XAVC compared to HD and 2K. I need to test this further to see what’s going on, but it’s not quite as perfect as perhaps hoped.

What we know: The hack tricks the camera into behaving like a PMW-F55. We know the F5 and F55 have different sensors with very different behaviour, different colour filters and different sensitivity, so the processing must be different for these two cameras.

My very quick test, shooting the same scene in 4K and in normal HD XAVC shows the 4K to be darker. Others are reporting that the 4K clips appear to over-exposure more readily than the HD.

SPECULATION: If the hack is doing more than just turning on 4K recording, if it is making the camera use the F55’s processing, then there will be a miss-match between the sensor and the processing and this might cause issues with dynamic range, gain and colour.

As soon as I get a chance (should be tomorrow) I will do some more involved testing to see exactly what is going on.

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Samyang launches 50mm Cine Lens

New Samyang 50mm T1.5 vDSLR lens.

New Samyang 50mm T1.5 vDSLR lens.

At long last Samyang have filled the gap in their vDSLR lens line up! It was crazy not to have a 50mm lens. Finally they are launching a 50mm T1.5 lens with pitch gears etc. This lens will be available next month (September) so not too long to wait. It’s full frame so should work great with the A7s as well as all your Super35 and APS-C cameras.

Hop over to the Samyang web site for sample images and further information. I have one on pre-order as as soon as I can I will check it out.

 

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Sony Launches New PXW-X500 XDCAM Shoulder Mount CCD camcorder with XAVC

New Sony PXW-X500 camcorder.

New Sony PXW-X500 camcorder.

Today Sony launched a couple of the new cameras. The MC2500E a very low cost, fairly basic shoulder mount AVCHD camcorder with a single 1/4″ CMOS sensor and a new full size, shoulder mount XDCAM camcorder the PXW-X500.

This is basically a replacement for the PMW-500 with the added benefit of the XAVC and SStP codecs. Like the PMW-500 and PDW-850 this is one of the few cameras to still use CCD sensors, so no flash bands or skew, making it a great news gathering workhorse. It has some new features not found on the PMW-500 including upto 120fps S&Q motion (looks like this is an option) and GPS. It looks like this possibly has the same very good sensors as the PMW-500 but with new signal processing and improved noise reduction. More details can be found on the Sony web site.

So really that just leaves the PMW-200 without XAVC. It can’t be long before we see a PMW-200 replacement with XAVC.

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Scheider Xenon FF Prime Lenses.

Schneider Xenon 50mm FF lens on the Sony A7s

Schneider Xenon 50mm FF lens on the Sony A7s

When you think of cine lenses then there are several brands that immediately come to mind. Zeiss, Arri, Cooke and Angenieux are probably the most familiar names but there are many others too. One brand I have been looking at more and more recently is Schneider.
Schneider Kreuznach have been making lenses since 1913. Based in Kreuznach in Germany they have long been know for their innovative designs and they won an Oscar in 2001 for Technical Achievement for their Super-Cinelux motion picture lenses.
A few years ago I met one of their lens engineers at NAB. I don’t think I have ever met a man as passionate about a lens design before or since. Every Schneider lens that I have ever used has been brilliant. They always seem to have near zero breathing, are always extremely solidly built and produce great images. So when I got a call from Manfrotto, the UK distributor to see if I would like a chance to play with some of the new Xenon FF (Full Frame) lenses I grabbed the opportunity.

Another view of the 50mm Schneider Xenon FF lens.

Another view of the 50mm Schneider Xenon FF lens.

The Xenon FF lenses are cine style lenses available with either Canon, Nikon or PL mounts. The mounts can be changed should you need to switch mounts at a later date. They are priced to directly compete with the Zeiss compact primes. At the moment there are only 3 lenses available, a 35mm, 50mm and 75mm, all are T2.1. In the near future there will also be a 25mm and 100mm T2.1 as well as an 18mm T2.4 Yum Yum! I’d love to have one of those for my Northern Lights or Storm Chasing expeditions. They are all the same size, have a 100mm front diameter, all have a 95mm lens thread. This means that swapping lenses during a shoot is straight forward as you don’t have to change Matte Box donuts or re-position the follow focus if you’re using one. Being Full Frame lenses and rated for 4K these should be a great match with the Sony A7s.

I got to play with a 35mm and 50mm with EF mount and decided to try them on my full frame A7s shooting in HD as well as taking a few still photos (which are the equivalent to 4.5K) on a cloudy and rainy day.

14 blade iris and EF mount on the 35mm Xenon FF.

14 blade iris and EF mount on the 35mm Xenon FF.

Straight out of the box you cannot help but be impressed by the build quality. These are substantial lenses, weighing in at around 1.25kg each with the EF mount. I could not find any plastic on these lenses, they look built to last.

The focus scale is large and easy to read, each lens being individually calibrated. Focus travel is a full 300 degrees. Even as you get to the far end of the focus ring the distances are still nicely spaced. From 9ft(3m) to infinity is around 100 degrees. Compare that to most DSLR lenses where the same focus range might be compressed into just  5 or 10 degrees and you can see that precise focus is much easier. Although sometimes  a very large focus travel can make focus pulls a little harder simply because or the large distance the focus ring has to be turned. But I’ll take a big focus throw lens over small throw any day.
The lenses have 14 curved iris blades giving a very round aperture even when stopped right down. I love peering into these lenses at the aperture blades as they are a work of art (but really hard to take a photo of). You can also see in the photo that the coatings of the lens are a distinct orange colour.

Photo taken with the 50mm Xenon FF.

Photo taken with the 50mm Xenon FF. Click on the image to enlarge or view at original resolution.

In practice the lenses did not disappoint. It did seem a bit odd to have such a large and heavy lens on the diminutive A7s, but as image quality starts with the lens a good lens can make all the difference. I shot at various apertures from wide open at T2.1 down to about T8 and didn’t notice any significant change in resolution across the range (I took photos as well as video to check the lens performance).

The lenses do tend to flare a little bit, the 35mm more than the 50mm, but I thought the flares were quite pleasing, others may disagree. Take a look at the video to get an idea of what they are like. There was a bit more flare at T2.1 compared to T2.8 or T4 on both lenses.
I did some big focus pulls to see how much breathing there was and as with all the Schneider lenses I’ve used breathing was very minimal. There is some breathing, these are not like the Cine-Xenars which have virtually zero breathing, but the breathing really is small.

Frame grab, shooting through trees. 35mm Xenon FF and A7s.

Frame grab, shooting through trees. 35mm Xenon FF and A7s.

Another test shot was to shoot some tree branches silhouetted against the sky to check for CA and colour fringing. Basically I can’t see any. Maybe right out in the very corners of the frame there is the tiniest bit of CA, but you really have to hunt for it.

Shopping basket frame grab. 35mm Xenon FF. I love the smoothness to the highlights.

Shopping basket frame grab. 35mm Xenon FF. I love the smoothness to the highlights.

Colour wise there is no obvious colour shift, if anything perhaps very, very slightly warm. As expected the lenses are very sharp and crisp, from corner to corner, but not excessively so. I found that the images contained a lot of detail but had a pleasing roundness too them that I really like. I shot a chrome shopping basket and the reflections of the bright chrome look really nice. I think this is a combination of a little bit of flare without excessive sharpness. I think it’s a very nice natural look. This can be one of the benefits of a video lens over a stills lens. Stills lenses must be incredibly sharp to work with 24 or 36 mega pixel sensors. Sometimes this results is a super sharp image that lacks character. Arguably if you start with a very sharp image you can always soften it a bit in post, but sometimes it’s nice to start off with a more rounded image. Look at how popular Cooke lenses are, they are well known for their rounded rather than super sharp images.

As expected from a 14 blade iris the bokeh is very creamy and smooth. Both near and far out of focus areas look very good indeed. Out of focus edges are smooth and don’t show any obvious double edges or other distortions.

It's really easy to get a very shallow DoF with a full frame sensor. Xenon FF 50mm and A7s.

It’s really easy to get a very shallow DoF with a full frame sensor. Xenon FF 50mm and A7s.

Take a look at the video for a better idea of the lens flares and the overall image quality. I really like the look you get from these lenses and wouldn’t mind a set of them for myself. I feel they have a lot in common with Cooke lenses, but at a much more affordable price. I hope to test them further in the near future and to a wider variety of scenes. I suspect they will be very good on skin tones and faces.

 

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Calling all XDCAM shooters – win a Sony Action Cam.

Sony are putting together an XDCAM demo reel to show on the Sony booth at IBC. They are looking for examples of great XDCAM footage to include in the reel. The footage should be inspirational, exciting, dramatic, romantic, happy, pretty, colourful. Basically anything eye catching or with a “Wow” factor.

Here is what Sony are asking for:

·         3-5 second emotional clips (the more clips the merrier).

·         Tone is emotional (fun, dangerous, could be sad but not too tragic), action, extreme – music on final project will be fast paced, action.

·         No political symbols/figures, no blood, no dead people.

·         It has to have been shot on a Sony XDCAM camera (and specify which one).

·         PMW-100/150/200/300/400/500

·         PXW-X180/X160

·         PDW-680/700/F800

·         These clips should be rights free, these clips will be posted on the various Sony digital channels and might be shown at Sony-run presentations – but they will not be distributed to anyone outside of Sony.

·         The clips can either be in MP4 or just a link for submission, and then if we choose to use your footage we’ll come back to you and ask for a higher quality format.

If your footage is chosen to feature in the video you will be eligible to win one of 30 Action Cams up for grabs on a first come first serve basis! (Please note that your footage may still be used after the 30 action cams have been given away. By submitting your footage you will be agreeing to these T&Cs).

As IBC is coming up very fast there is some urgency to get this footage. If you have anything that you wish to submit please send a link to the clip to:

Lauren(DOT)Brogden(AT)eu(DOT)sony(DOT)com

REPLACE (DOT) with . and (AT) with @

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Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.

This document has been prepared independently of Sony. It is based on my own findings having used the camera and tested various exposure levels and methods. LUT’s to accompany this article can be found here.

If you find this useful please consider buying me a coffee or a beer. I’m not paid to write these articles.


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One of the really nice features of the Sony A7s is the ability to use different gamma curves and in particular the Sony S-Log2 gamma curve.

What are gamma curves?

All conventional cameras use gamma curves. The gamma curve is there to make the images captured easier to manage by making the file size smaller than it would be without a gamma curve. When TV was first developed the gamma curve in the camera made the signal small enough to be broadcast by a transmitter and then the gamma curve in the TV set (which is the inverse of the one in the camera) expanded the signal back to a normal viewing range. The current standard for broadcast TV is called “Recommendation BT-709″, often shortened to Rec-709. This gamma curve is based on standards developed over 60 years ago and camera technology has advanced a lot since then! Even so, almost every TV and monitor made today is made to the Rec-709 standard or something very similar. Many modern cameras can capture a brightness range, also known as dynamic range, that far exceed the Rec-709 standard.

The limitations of standard gammas.

As gamma effects the dark to light range of the image, it also effects the contrast of the image. Normal television gamma has a limited dynamic range (about 6 to 7 stops) and as a result also has a limited contrast range.

When shooting a high contrast scene with conventional gamma the brightest highlights and the darkest shadows cannot be recorded.

When shooting a high contrast scene with conventional gamma the brightest highlights and the darkest shadows cannot be recorded. The contrast on the TV or monitor will however be correct as the camera captures the same contrast range as the monitor is able to display.

Normally the gamma curve used in the camera is designed to match the gamma curve used by the TV or monitor. This way the contrast range of the camera and the contrast range of the display will be matched. So the contrast on the TV screen will match the contrast of the scene being filmed and the picture will look “normal”. However the limited dynamic range may mean that very bright or very dark objects cannot be accurately reproduced as these may exceed the gammas dynamic range.

Although the dynamic range of Rec-709 may not always capture the entire range of the scene being shot, as the gamma of the camera matches the gamma of the TV the contrast will appear correct.

Although the dynamic range of Rec-709 may not always capture the entire range of the scene being shot, as the gamma of the camera matches the gamma of the TV the contrast will appear correct.

The over exposure typical of a restricted range gamma such as Rec-709  is commonly seen as bright clouds in the sky becoming white over exposed blobs or bright areas on faces becoming areas of flat white. Objects in shade or shadow areas of the scene are simply too dark to be seen. But between the overexposed areas and any under exposure the contrast looks natural and true to life.

Typical limited Rec-709  exposure range. Contrast is good but the clouds are over exposed and look un-natural.

Typical limited Rec-709 exposure range. Contrast is good but the clouds are over exposed and look un-natural.

Log Gamma.

Log gamma, such as Sony’s S-Log2, allows the camera to capture a much greater brightness range or dynamic range than is possible when shooting with conventional television gamma. Dynamic range is the range from light to dark that the camera can capture or the range that the monitor or TV can display within one image. It is the range from the deepest blacks to the brightest whites that can be captured or shown at the same time.

There are some things that need to be considered before you get too excited about the possibility of capturing this much greater dynamic range. The primary one being that if the camera is set to S-log2 and the TV or monitor is a normal Rec-709 TV (as most are) then there is no way the TV can correctly display the image being captured, the TV just doesn’t have the range to show everything that the camera with it’s high range log gamma can capture accurately.

Fixed Recording Range For Both Standard and Log Gamma.

The signal range and signal levels used to record a video signal are normally described in percent. Where black is 0% and the brightest thing that can be recorded is normally recorded at 100% to 109%. Most modern video cameras actually record the brightest objects at 109%. The important thing to remember though is that the recording range is fixed. Even when you change gamma curve the camera is still constrained by the zero to 109% recording range. The recording range does not change whether you are recording Rec-709 or S-log2. So log gamma’s like S-Log2 must squeeze a much bigger signal range into the same recording range as used by conventional Rec-709 recordings.

Log gamma squeezes the scenes large range to fit in the camera's normal 0%-109% recording range.

Log gamma squeezes the scenes large range to fit in the camera’s normal 0%-109% recording range.

Recording S-Log2.

In order to record using S-log2 with the A7s you need to use a picture profile. The picture profiles give you several recording gamma options. For S-log2 you should use Picture Profile 7 which is already set up for S-log2 and S-Gamut by default (for information on gamuts see this article). In addition you should ALWAYS use the cameras native ISO which is 3200 ISO and it is normally preferable to use a preset white balance. Using any other ISO with S-log2 will not allow you to get the full benefit of the full 14 stops of dynamic range that S-log2 can deliver.

Grey Cards and White Cards.

Before I go further let me introduce you to grey and white cards in case you have not come across them before. Don’t panic you don’t have to own one, although I would recommend getting a grey card such as the Lastolite EzyBalance if you don’t have one. But it is useful to understand what they are.

The 90% White Card.

The 90% white card is a card or chart that reflects 90% of the light falling on it. This will be a card that looks very similar in brightness to a piece of ordinary white paper, it should be pure white, some printer papers are bleached or coloured very slightly blue to make them appear “brilliant white”  (as you will see later in many cases it is possible to use an ordinary piece of white paper in place of a 90% white card for exposure).

The Grey Card.

The 18% grey card, also often called “middle grey” card, is a card that reflects 18% of the light falling on it. Obviously it will appear much darker than the white card. Visually to us humans an 18% grey card appears to be half way between white and black, hence it’s other name, “middle grey”.

Middle grey is important because the average brightness level of most typical scenes tends to be around the middle grey brightness value. Another key thing about middle grey is that because it falls in the middle of our exposure range it makes it a very handy reference level when measuring exposure as it is less likely to be effected by highlight compression than a 90% white card.

Exposing White and Middle Grey.

Coming back to Rec-709 and conventional TV’s and monitors. If we want a piece of white paper to look bright and white on a TV we would record it and then show it at somewhere around 85% to 95% of the screens full brightness range. This doesn’t leave much room for things brighter than a white piece of paper! Things like clouds in the sky, a shiny car, a bright window or a direct light source such as a lamp or other light.  In order to make it possible for S-log2 to record a much greater dynamic range the recording level for white and mid tones is shifted down. Instead of recording white at 85%-95%, when using S-log2 it is recommended by Sony that white is recorded at just 59%. Middle grey moves down too, instead of being recorded at 41%-42% (the normal level for Rec-709) it’s recorded at just 32%. By recording everything lower this means that there is a lot of extra space above white to record all those bright highlights in any scene that would be impossible to record with conventional gammas.

To make room for the extra dynamic range and the ability to record very bright objects, white and mid tones are shifted down in level.

To make room for the extra dynamic range and the ability to record very bright objects, white and mid tones are shifted down in level by the S-log2 gamma curve. As a result, white, mid tones etc will be displayed darker than normally expected with conventional gamma.

As S-Log2 normally shifts a lot of the recording levels downwards, if we show a scene shot with S-Log2 that has been exposed correctly on a conventional Rec-709 TV or monitor it will look dark due to the lower recording levels. In addition it will look flat with very low contrast as we are now squeezing a much bigger dynamic range into the limited Rec-709 display range.

The on screen contrast appears reduced as the capture contrast is greater than the display contrast.

The on screen contrast appears reduced as the capture contrast is greater than the display contrast.

This on screen reduction in contrast and the darker levels are actually perfectly normal when shooting using log gamma, this is how it is supposed to look on a normal monitor or TV. So don’t be alarmed if when shooting using S-Log2 your images look a little darker and flatter than perhaps you are used to when shooting with a standard gamma. You will adjust the S-Log2 footage in post production to restore the brightness and contrast later.

Correctly exposed S-Log2 can look dark and washed out.

Correctly exposed S-Log2 can look dark and washed out.

The post production adjustment of S-Log2 is very important and one of the keys to getting the very best finished images. The S-Log2 recording acts as a digital negative and by “processing” this digital negative in post production (normally referred to as “grading”) we manipulate the large 14 stop dynamic range of the captured image to fit within the limited display range of a Rec-709 TV in a pleasing manner. This may mean pulling up the mid range a bit, pulling down the highlights and bit and generally shifting the brightness and colour levels of different parts of the image around  (see PART 2 for more post production information).

SLog-2 and 10 bit or 8 bit data.

Originally Slog-2 was designed for use on high end digital cinema cameras such as Sony’s F65 camera. These cameras have the ability to record using 10 bit data. A 10 bit recording can have up to around 1000 shades of grey from black to white. The A7s however uses 8 bit recording which only has a maximum of 235 shades from black to white. Normally 8 bit recording is perfectly OK as most transmission and display standards are also 8 bit. Shoot with an 8 bit camera and then display that image directly via an 8 bit system and nothing is lost. However when you start to grade and manipulate the image the difference between 8 bit and 10 bit becomes more significant. If you start to shift levels around, perhaps stretching out some parts of the image then the increased tonal resolution of a 10 bit recording helps maintain the very highest image quality. Photographers that have shot using both jpeg and raw will know how much more flexibility the 12 bit (or more) raw files have compared to the 8 bit jpeg’s. However they will also know that 8 bit jpeg’s can be also adjusted, provided you don’t need to make very large adjustments.

Contrary to popular belief heavy grading 8 bit footage does not necessarily lead to banding in footage across smooth surfaces except in extreme cases. Banding is more commonly a result of compression artefacts such as macro blocking. This is especially common with very highly compressed codecs such as AVCHD. The 50Mbps XAVC-S codec used in the A7s is a very good codec, far superior to AVCHD and as a result compression artefacts are significantly reduced, so banding will be less of an issue than with other lower quality codecs. If your going to shoot using S-Log2, some grading will be necessary and as we only have 8 bit recordings we must take care to expose our material in such a way as to minimise how far we will need to push and pull the material.

Getting Your Exposure Right.

When S-Log2 was developed the engineers at Sony produced tables that specified the correct exposure levels for s-Log2 which are:

exposure table1As you can see the nominal “correct” exposure for S-Log2 is a lot lower than the levels used for display on a typical Rec-709 TV or monitor. This is why correctly exposed s-log2 looks dark on a conventional TV. The implication of this is that when you grade your footage in post production you will have to shift the S-log2 levels up quite a long way. This may not be ideal with an 8 bit codec, so I decided to carefully test this to determine the optimum exposure level for the A7s.

Correct Exposure.

The panel of images below is from the A7s recording S-log2 and exposed at the Sony recommended “correct” 32% middle grey level. The correct exposure was determined using a grey card and an external waveform monitor connected to the cameras HDMI output. Then the S-log2 was corrected in post production to normal Rec-709 levels using a Look Up Table (LUT – more on LUT’s in part 2). You can also see the viewfinder display from the camera. If you click on the image below you can expand it to full size. Sorry about the shadow from the laundry line, I didn’t see this when I was shooting the test shots!

Correctly exposed S-Log2 from A7s.

Correctly exposed S-Log2 from A7s.

From this you can see just how dark and low contrast looking the original correctly exposed S-log2 is and how much more vibrant the corrected Rec-709 image is. I have also indicated where on the cameras histogram middle grey and white are. Note how much space there is to the right of white on the histogram. This is where the extra highlight or over exposure range of S-log2 can be recorded. When correctly exposed S-log2 has an exposure range of 6 stops above middle grey and 8 stops under.

Over Exposing or “Pushing” S-log2.

If we deliberately raise the exposure level above the Sony recommended levels (known as pushing the exposure), assuming you grade the image to the same final levels some interesting things happen.

For each stop we raise the exposure level you will have 1 stop (which is the same as 6db) less noise. So the final images will have half as much noise for each stop up you go. This is a result of exposing the image brighter and as a result not needing to raise the levels in post as far as you would if exposed at the normal level.

You will loose one stop of over exposure headroom, but gain one stop of under exposure headroom.

Bright highlights will be moved upwards into the most compressed part of the log gamma curve. This can result in a loss of texture in highlights.

Skin tones and mid tones move closer to normal Rec-709 levels, so less manipulation is need for this part of the image in post production.

This last point is important for the A7s with it’s 8 bit codec, so this is the area I looked at most closely. What happens to skin tones and textures when we raise the exposure?

Exposing at +1, +2 and +3 Stops.

Below are another 3 panels from the A7s, shot at +1 stop, +2 stops and +3 stops. Again you can click on the images if you wish to view them full size.

A7s S-Log2 over exposed by one stop.

A7s S-Log2 over exposed by one stop.

A7s S-Log2 over exposed by 2 stops.

A7s S-Log2 over exposed by 2 stops.

A7s S-Log2 over exposed by 3 stops.

A7s S-Log2 over exposed by 3 stops.

Looking at these results closely you can see that when you increase the exposure by 1 stop over the Sony specified correct level for S-log2 there is a very useful reduction in noise, not that the A7s is particularly noisy to start with, but you do get a noticeably cleaner image.

Below are 4 crops from the same images, after grading. I really recommend you view these images full size on a good quality monitor. Click on the image to view larger or full size.

Crops at different exposure of LUT corrected A7s S-log2 footage.

Crops at different exposure of LUT corrected A7s S-log2 footage.

The noise reduction at higher exposures compared to the base exposure is very clear to see if you look at the black edge of the colour checker chart (the coloured squares), although the difference between +2 and +3 stops is very small. You can also see further into the shadows in the +3 stop image compared to the base exposure. A more subtle but important effect is that as the exposure goes up the visible texture of the wooden clothes peg decreases. The grain can be clearly seen at the base level but by +3 stops it has vanished. This is caused by the highlights creeping into the more compressed part of the log gamma curve. The same thing is happening to the skin tones in the +3 stop image, there is some reduction of the most subtle textures.

From this we can see that for mid tones and skin tones you can afford to expose between 1 and 2 stops above the Sony recommended base level. More than 2 stops over and brighter skin tones and any other brighter textures start to be lost. The noise reduction gain by shooting between one and 2 stops over is certainly beneficial. The down side to this though is that we are reducing the over amount of exposure headroom.

As you raise the exposure level you reduce the over  exposure headroom.

As you raise the exposure level you reduce the over exposure headroom.

Given everything I have seen with this 8 bit camera my recommendation is to shoot between the Sony recommended base S-log2 level and up to two stops over this level. I would try to avoid shooting more than 2 stops over as this is where you will start to see some loss of texture in brighter skin tones and brighter textures.  Exactly where you set your exposure will depend on the highlights in the scene. If you are shooting a very bright scene you will possibly need to shoot at the Sony recommended level to get the very best over exposure headroom. If you are able to expose higher without compromising any highlights then you should aim to be up to 2 stops over base.

Determining The Correct Exposure.

The challenge of course is determining where your exposure actually is. Fortunately as we have seen, provided you in the right ball park, S-log2 is quite forgiving, so if you are a little bit over exposed it’s probably not going to hurt your images much. If you have a waveform monitor then you can use that to set your exposure according to the table below. If you don’t have proper white or grey cards you can use a piece of normal white paper. Although slightly less accurate this will get you very close to where you want to be. Do note that white paper tends to be a little brighter than a dedicated 90% reflectivity white card. If you don’t have any white paper then you can use skin tones, again a bit less accurate but you should end up in the right zone.

My suggested exposure levels for the Sony A7s. The "sweet spot" is from normal to +2 over.

My suggested exposure levels for the Sony A7s. The “sweet spot” is from normal to +2 over.

If you don’t have an external waveform monitor then you do still have some good options. Sadly although the camera does have zebras, these are not terribly useful for S-log2 as the lowest the zebras can go is 70%.

Light Meter: You could use a conventional photography light meter. If you do choose to use a light meter I would recommend checking the calibration of the light meter against the camera first.

Mark 1 Eyeball: You could simply eyeball the exposure looking at the viewfinder or rear screen but this is tricky when the image is very flat.

In Camera Metering: The cameras built in metering system, like the majority of DSLR’s is calibrated for middle grey. By default the camera uses multi-point metering to measure the average brightness of several points across the scene to determine the scenes average brightness and from there set the correct base S-log2 exposure.

Auto Exposure:

When you are using S-Log2, auto exposure in most cases will be very close to the correct base exposure if you use the default Multi-Zone exposure metering. The camera will take an average exposure reading for the scene and automatically adjust the exposure to the Sony recommended 32% middle grey exposure level based on this average. In the P, A and S modes you can then use the exposure compensation dial to offset the exposure should you wish. My recommendation would be to add +1 or +2 stops via the dial. Then observe the histogram to ensure that you don’t have any significant over exposure. If you do then reduce the exposure compensation. Lots of peaks to the far right of the histogram is an indication of over exposure.

Manual Exposure And Internal Metering.

If you are exposing manually you will see a small M.M. indication at the bottom of the LCD display with a +/- number. In the eyepiece viewfinder this appears as a scale that runs from -5 to +5, in S-log2 only the -2 to +2 part of the scale is used. In both cases this is how far the camera thinks you are away from the optimum exposure. + meaning the camera is over exposed, – meaning under.

A7s Viewfinder indications in manual exposure mode showing both M.M. offset from metered exposure and histogram.

A7s Viewfinder indications in manual exposure mode showing both M.M. offset from metered exposure and histogram.

In the image above we can see the M.M. indication is +0.3, in the eyepiece you would see a small arrow one bar to the right of “0” , indicating the cameras multi zone metering thinks the shot is just a little over exposed, even though the shot has been carefully exposed using a grey card and external waveform monitor. This error is probably due to the large amount of white in the shot, white shirt, white card, test charts with a lot of brighter than grey shades.  In practice an error of 0.3 of a stop is not going to cause any real issues, so even if this was exposed by setting  the exposure so that you have “M.M. 0.0″ the exposure would be accurate enough. But it shows that multi point exposure averaging is easily confused.

The scene above is a fairly normal scene, not excessively bright, not particularly dark. If shooting a snow scene for example the cameras multi point averaging would almost certainly result in an under exposed shot as the camera attempts to bring the bright snow in the scene down to the average middle grey level. If shooting a well lit face against a very dark background then the averaging might try to bring the background up and the shot may end up overexposed.

If you want really accurate exposure then you should put the cameras metering system into the spot metering mode where instead of taking an average of various points across the scene the camera will just measure the exposure at the very center of the image.

A7s Spot Metering Mode.

A7s Spot Metering Mode.

You can then use a grey card to very accurately set the exposure. Simply place the circular shaped symbol at the center of the viewfinder display over a grey card and set the exposure so that M.M is 0.0 for the correct S-Log2 base exposure. To expose 1 stop over with a grey card, set M.M. +1.0 and two stops over M.M. +2.0 (not flashing, flashing indicates more than +2 stops).

Using Spot Metering to set exposure correctly for S-log2. MM 0.0.

Using Spot Metering to set exposure correctly for S-log2. MM 0.0.

One small issue with this is that the camera will only display a M.M. range of -2.0 to +2.0 stops. Provided you don’t want to go more than 2 stops over base then you will be fine with a grey card.

Using White Instead of Grey:

If you don’t have a grey card then you can use a 90% reflectivity white target. As white is 2 stops brighter than middle grey when S-Log2 is correctly exposed the 90% white should indicate M.M +2.0.

Using spot metering to set the correct exposure for S-Log2. M.M should read M.M +2.0 for a 90% reflectivity white target.

Using spot metering to set the correct exposure for S-Log2. M.M should read M.M +2.0 for a 90% reflectivity white target.

Once you have established the correct exposure you can then open the iris by 1 or two stops to increase the exposure. Or halve the shutter speed to gain a one stop brighter exposure. Each time you halve the shutter speed your exposure becomes one stop brighter, so divide the shutter speed by 4 to gain a 2 stop increase in exposure. As always you should observe the histogram to check for any over exposure. White peaks at the far right of the histogram or disappearing completely off the right of the histogram is an indication of over-exposure. In this case reduce your exposure back down towards the base exposure level (M.M 0.0 with a a grey card).

Exposure Summary:

I recommend using an exposure between the “correct” base S-Log2 exposure level of middle grey at 32% and two stops over this. I would not recommend going more than 2 stops over over base.

In the P, A and S auto exposure modes, when using the default multi-zone metering the camera will set the base S-log2 exposure based on the average scene brightness. For most typical scenes this average should be very close to middle grey. This exposure can then be increased (brightened) by up to 2 stops using the exposure compensation dial.

In manual exposure the “M.M.” number displayed at the bottom of the viewfinder display is how far you are from the correct base S-log2 exposure. M.M. +2.0 indicates +2 stops over base. If using multi zone metering (the cameras default) this exposure will be based on the scenes average brightness.

If you set the metering to “Spot” you can use a grey card centred in the image to determine the correct base exposure and up to 2 stops of over exposure via the M.M. indication when shooting manually.

In Part 2:

In part two I will take a look at grading the S-log2 from the A7s and how to get the very best from the S-log2 images by using Look Up Tables (LUT’s).

I welcome feedback on my articles. If you have any feedback please let me know. I will make this available as a PDF for download once part 2 is completed.

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